In modern Direct Sequence Spread Spectrum (DSSS) modulators, data is convolutionally or turbo encoded, punctured, and interleaved prior to transmission. The Telecommunication Industry Association (TIA) IS-2000 standard outlines two interleaving requirements, one for backward compatibility with previous DSSS standards, and a second for newly introduced data rates and channel structures. The IS-2000 standard also requires puncturing of 20 millisecond (ms) frames with 5 ms or 10 ms frames.
“Puncturing” in the current context means replacing some portion of a 20 ms frame with a shorter length (5 ms or 10 ms) frame. The portion may be any one of the four possible quadrants of a 20 ms frame (5 ms puncturing), or one of either the first or the second halves of a 20 ms frame (10 ms puncturing). In either case, more than one interleaver configuration must be employed (one for the original 20 ms frame and one for the punctured frame). The interleaving mechanism for IS-2000 is given in the standard as a read-address generator, implying the function would be implemented at the output or “read” side of the interleaver memory. Doing this would cause the output address control circuitry to be unnecessarily complex since one interleaving engine with a dedicated memory per data channel would be necessary to accomplish the puncturing.
IS95 interleaver designs include a two-stage approach for interleaving data. First a write function is used to place data in memory, then a specialized read function is used to access the data for transmission. A traditional modulator 100 configuration is shown in FIG. 1. A write address generator 102 is used to place data in memory 106, while a read address generator 104 is used to access data from memory 106.
The modulator architecture shown in FIG. 1 may work for the IS-2000 standard, but substantial control circuitry will be required to orchestrate the puncturing of sub-20 ms frames on the output side of the memory. The IS-2000 complexity is increased because multiple interleaver engines are needed to build individual channels when puncturing of 20 msec frames occurs. A need thus exists in the art for a method and apparatus that can handle transmission data in the modulator and that can support a variety of interleaver structures and perform efficient puncturing of frames.